Why Are There Trees in This Parking Lot Instead of More Parking Spots?

2010 December 16

Do you ever drive to the store only to find you can’t find a spot to park in? It’s bad enough that prime parking spots are taken up by Cart Corrals or car-share companies—but now more new parking lots are being designed with planted sidewalk areas in place of more parking spots. Yeah, the trees are nice, cleaning up the air and all that, and the shrubs and flowers are pretty, but what else are they good for? A lot, actually, in terms of stormwater management.

You know how you walk through a parking lot in the rain and the water either pools into the only convenient areas to walk safely through said parking lot, or there’s hardly any water on the ground anywhere? An incorrectly graded parking lot allows the water to pool, while a correctly graded parking lot will drain the water right away. In that correctly graded parking lot, the water drains right into the storm drain—and discharges directly without treatment to local waters. While water is running though the parking lot, it picks up dirt, trash, and other pollutants, including motor oil, antifreeze, and air conditioner condensate. These pollutants also go into the local waters.

One way to tackle this stormwater problem is to design new parking lots with planting areas. If you’re working with an existing parking lot, you can do something called “retrofitting.” Retrofitting is the practice of upgrading an existing area using new technologies that were not available when the area was initially developed. If a parking lot is being redone, or a township is looking for ways to decrease the amount of impervious surfaces in their area, planting beds are a great way to reduce impervious surfaces and increase stormwater infiltration, retention, and evaporation.

Choosing certain kinds of plants and or trees will not only promote infiltration of water into soil, but the plants will also hold water on their leaves to then evaporate back into the atmosphere at a later time, keeping water off the ground in the first place. Although planting beds have to be watered when precipitation events do not occur, the beds are oftentimes mulched, which means more water retention and less watering in the interim. And not to make light of the litter problem, but planting beds and trees also help catch rough trash that travels in the wind. Trash captured by these planting beds doesn’t end up in the streams of water that enter storm drains, and the beds are easy to clean up and remove the trash from the environment.

You can also retrofit at home. If you are lucky enough to have a driveway, the next time it has to be redone, think about tearing it up and putting a pervious surface down. Pervious surfaces are those that allow infiltration of stormwater, in contrast to impervious surfaces which promote storm water runoff. There are lots of pervious options to choose from, including vegetation (yes, vegetation!), paving blocks, bricks, permeable clay, crushed organic matter, and aggregate/gravel (which can be made from recycled asphalt—i.e., the pavement you just tore up). All of these materials reduce the amount of water that runs into storm drains and increases infiltration—which means less pollution and a cleaner environment.

Do you know of any examples of retrofitting in your area? Have you retrofitted your driveway without even knowing it? Are there any volunteer opportunities out there to help retrofit storm basins in your community? If so, let us know! We’d love to hear from you.

Editor's Note: The opinions expressed here are those of the author. They do not reflect EPA policy, endorsement, or action, and EPA does not verify the accuracy or science of the contents of the blog.

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